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A Links –Link between an end point and an STP –“A” stands for Access –Purpose is to deliver signaling messages –Connects an SSP to an STP –Connects an SCP to an STP C Link –Link between 2 STPs (making them a “mated” pair) –“C” stands for Cross Link SS7 Link Types 5

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Used to idle the signaling link and keep track of transmitted and received frames Even though HDLC framing is used on signaling linking frames; MTP does not allow for more then 2-3 idle flags to be transmitted, instead FISU frames are transmitted repeatedly FISU Frames 16

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Used to sync the state machines on both sides of the link Frame length can be 1 or 2 (normally 1 and normally switches are supposed to be compatible with both) 6 types of frames identified by the value of the status field (Status Indication X) Service Information Octet (ITU Q.703, , page 23): –0, SIO, “out of alignment” –1, SIN, “normal alignment” –2, SIE, “emergency alignment” –3, SIOS, “out of service” –4, SIPO, “processor outage” –5, SIB, “busy” LSSU Frames 17